1. Technical Field of the Disclosure
The present invention relates to an electromagnetic flow meter, and more particularly an electromagnetic flow meter having a grounding means improved and adapted for metering the flow of a highly corrosive liquid.
2. Description of the Prior Art
In the field of electromagnetic flow meters, it is known to measure the flow of an electrically conductive or semi-conductive liquid in a pipeline by utilizing the phenomenon that an electric conductor moving in a magnetic field induces an electromotive force, the value of which is proportional to the movement of the conductor. In known devices based on this principle, the liquid flows through a pipeline placed in a uniform magnetic field, so that its axis is at right angles to the lines of force of the field. Arranged diametrically opposite to one another in the wall of the pipeline are two electrodes, so that the connecting pipeline is about at right angles to the lines of force. Due to the flow of the liquid, a voltage is induced between the electrodes, which is substantially proportional to the flow of the liquid and which is measured.
In the electromagnetic flow meter, a grounding means is indispensable from the standpoint of improving the electrical characteristics. Effective grounding of the electromagnetic flow meter is necessary for maximum accuracy. If the connecting pipelines are provided with corrosion-resistant internal coats or linings, or are completely of plastic, the connection to the counterflanges fails to achieve reliable grounding. A grounding means or grounding ring must be mounted on the inlet or outlet side of the electromagnetic flow meter body. The grounding ring, that is, the ring-shaped liquid-contacting electrode, comprises a grounding plate and a liquid-contacting electrode and is frequently sandwiched between the electromagnetic flow meter body and a mating pipe section of some plant pipeline.
Now, the grounding ring may be made of SUS 304 or 316 stainless steel, which is relatively inexpensive, in the case where the fluid to be metered is water or a slightly corrosive liquid. In case the fluid to be metered is a highly corrosive liquid such as potassium hydroxide (KOH), sodium hydroxide (NaOH), sulfuric acid (H.sub.2 SO.sub.4) or hydrochloric acid (HCl), however, it is difficult to use the SUS 316 stainless steel, as will be explained in the following Table, so that an extremely expensive material such as platinum-iridium, platinum or tantalum, which is highly corrosion resistant, is frequently used:
______________________________________ Name of Concen- Material Chemical tration Temperature Pt--Ir Ir Ta SUS316 ______________________________________ HCl 10% Room A A A C 20% Temp. 35% Boiling Point C A A C H.sub.2 SO.sub.4 10% Room A A A A 60% Temp. 80% 5% Boiling A A A C 95% Point HNO.sub.3 10% Room A A A A 30% Temp. 60% 10% Boiling A A A C 30% Point 60% Aqua HCl:3 Room C C A C Regia HNO.sub.3 :1 Temp. Caustic 20% Room Temp. A A B B Soda 20% Boiling Point A A C B 40% Room Temp. Boiling Point A A C -- ______________________________________
In the above Table, letters A, B and C designate "completely corrosion resistant", "usable" and "corroded", respectively. The platinum-iridium is a mixture containing 80 wt. % of platinum and 20 wt. % of iridium.
According to the prior art, the grounding ring has to be made of an expensive material from the standpoint of corrosion resistance. Accordingly, the propriety of the material has to be taken into consideration in view of the amount of the material used and the construction of the grounding ring. In short, the ring-shaped, liquid-contacting electrode according to the prior art cannot avoid an increase in its production cost and is not proper for industrial application because it requires a large amount of expensive material. In one of the liquid-contacting electrodes according to the prior art, on the other hand, screw-shaped electrodes made of an expensive material are used together with a ring of an insulating material. The screw-shaped electrodes are made of platinum-irridium, tantalum or the like. The reason why the platinum-iridium is used in place of platinum is that the corrosion resistance of platinum is superior to the platinum-iridium, but pure platinum is too soft to be used in the screw-type electrode. Here, the platinum-iridium has its hardness augmented by mixing platinum with about 20 wt. % of iridium, but it suffers several defects, namely, that it is inferior in its corrosion resistance to platinum and that it is far more expensive than platinum. For example, in case of using the screw-type platinum-iridium, the amount of the material is about 32 g for 8 electrodes.